The present invention relates to a supervisory circuit associated with optical repeaters of an optical repeating system for detecting control signals which are superposed on data to monitor the operations of the repeaters while the system is in service.
A prerequisite with long-haul optical repeating systems, especially undersea optical repeating systems, is that a terminal be provided with remote controlling and monitoring capabilities for monitoring the operations of the repeating paths, localizing failures, switching a redundancy system, etc. Since assigning an exclusive channel independent of transmission channels for a remote monitoring purpose is disadvantageous for economy reasons, it has been customary to superpose low-speed control signals on transmission path code sequences in the format of presence/absence of parity violations. Such an approach allows control signals to be transmitted even during communication by being superposed on data. A supervisory circuit which may be associated with these types of optical repeaters for detecting the control signals is taught by, for example, C. D. Anderson et al. in "The SL Supervisory System", IEEE Journal on selected areas in communications, VOL SAC-2, No. 6, Nov. 1984, pp. 1031-1037. As this paper indicates in FIG. 8, a conventional repeater supervisory circuit regenerates control signals by detecting bursts which appear in response to parity violations in the transmission path code sequences.
Specifically, a transmission path code is implemented with a 24BlP block format, i.e., a format wherein even parity bit is added to twenty-four bits of data to constitute a 25-bit block. The even parity is replaced with an odd parity every N blocks (N =224 in the above-mentioned paper) only during the mark period of a low-speed (33 1/3 baud rate) binary control signal. Data in the code being propagated over the path has been scrambled and randomized. Hence, when no control signal is superposed on data, a block with an even parity continuously appears with the result that the average DC level of codes on the path remains substantially constant throughout the blocks. While a control signal is superposed on data, a block with an odd parity appears at a predetermined period (25 .times. N bits) during the mark period of the control signal. Every time a block with an odd parity appears, the average DC level increases or decreases to produce an AC component in a burst. In the paper, since the rate (fb) of the transmission path code sequence is 295.6 Mbps, the frequency fv of the bursts is nearly equal to 26.4 kHz, i.e. fv =fb/2(2 .times.25 .times.N).
The conventional supervisory circuit separates the above-stated burst by a bandpass filter whose center frequency is fv, amplifies and rectifies the separated burst, passes the amplified and rectified burst through a low pass filter to detect a control signal, and then causes a supervisory logic circuit to process the control signal. When a control signal appears almost simultaneously on an up-going and a down-going channel, the supervisory logic circuit selects a control signal arrived first and inhibits the input of the other control signal until the above-mentioned sequence of steps completes.
A problem with the conventional repeater supervisory circuit described above is, as follows. Assume that the noise level has increased due to a fault occurred on the up-going channel. As the component having the frequency fv and contained in the noise becomes great, the supervisory logic circuit inhibits the input of a control signal from the down-going channel and thereby makes the access of an originating terminal transmitting a control signal impossible. For example, when the up-going channel is shut off, a photodetector receiving an optical signal from the up-going channel is caused into a no-input state with the result that microplasma noise being generated by the repeater sharply increases. In response, the component having the frequency fv and arriving at the supervisory circuit assigned to the up-going channel also increases. Then, it is determined that a control signal on the up-going channel has arrived, resulting in the erroneous operation as stated above.